One device of the above kind is described in U.S. Pat. No. 4,433,770 and in the corresponding French published patent application No. FR 2 495 255A. In that document, one of the coaxial parts comprises a hub and the other comprises two guide rings which are disposed on either side of a damper plate. The damper plate is fixed with respect to the hub, for limited rotation with respect to the latter through a loose meshing means. The loose meshing means comprise two co-operating sets of teeth defining a circumferential clearance between them. The intervention member is mounted rotatably and is adapted to come into engagement on the outer periphery of the hub, which constitutes in this connection a fixed abutment for the intervention member. A return spring is arranged to bear on the latter.
In a first range of relatively low values of rotational velocity of the assembly, all of these arrangements allow the resilient members of lesser stiffness which form part of the torsion damping device to act by themselves or to render these resilient members inoperative. The intervention member thus enables those noises which occur when the rotational velocity is below the slow running speed of the engine to be eliminated. This is the case for example when, with the engine rotating, declutching is followed by re-engagement of the clutch.
In a development of the above principle, a retractable abutment element is interposed on the path of rotation of the intervention member, for the temporary retention of the intervention member, in such a way that the latter may occupy one or other of two working positions, namely an intermediate position and a terminal position. In the intermediate position, which the intervention member occupies in a second range of values of rotational velocity of the assembly, greater than those of the above mentioned first range, the intervention member is in engagement against the retractable abutment element; and in this position it has no effect on the operating characteristics of the torsion damping device.
The intervention member can occupy its terminal position after the retractable abutment element has been withdrawn. In this terminal position, which corresponds to a third range of values of rotational velocity greater than that of the second range of values, the intervention member acts once again to modify the operating characteristics of the device.
The intervention member thus acts to eliminate not only "clunk" noises, but also out of balance phenomena which occur during running, in particular when the driver releases his foot from the accelerator and then depresses it again.
However, these arrangements are quite complicated, because they call for a rotatable mounting for the intervention member. In addition, this member is arranged outside the volume delimited by the guide rings. Accordingly, the assembly occupies considerable axial space; in addition, the intervention member is susceptible to being damaged before the torsion damper has been mounted in the vehicle, for example during warehousing and/or careless handling.